Measurement of blood pH is important in the critical care environment. Commercially available blood gas instruments measure pH, PO.sub.2 and pCO.sub.2. These instruments require either the extraction of a blood sample for in vitro instruments or the use of an invasive sensor for in vivo measurements. A non-invasive measurement of blood pH is desirable because it would eliminate the necessity of sample extraction or an invasive sensor, and would allow for continuous monitoring.
Hemoglobin oxygen saturation has been non-invasively monitored by means of pulse oximetry. Although widely used in the United States and elsewhere, these instruments have been limited to measuring oxygen saturation and have not provided information regarding blood pH.
U.S. Pat. No. 5,355,880 (the "'880 patent"), assigned to Sandia Corporation, presents a generalized discussion of the non-invasive (optical) measurement of blood gas parameters, including pH. Significantly, with regard to the present invention, this patent teaches away from using wavelengths in the 520-680 nm range. Thus, although the patent broadly discloses the use of wavelengths in the 500 nm to 2500 nm range to measure pH, PCO.sub.2, HCO.sub.3.sup.- !, PO.sub.2, and O.sub.2 saturation, in its examples it uses wavelengths in the 640-970 nm range for its multiple linear regression and best ratio analyses ('880 patent at column 24, lines 18-20 and 48-51), and in the 700-800 nm range for its partial least squares analysis ('880 patent at column 25, line 28). Further, in FIG. 10, the patent presents correlation coefficients for pH only for wavelengths in the 650-975 nm range. Along these same lines, the patent describes the prior art spectroscopic determination of pH in non-biological systems as using wavelengths in the 1100-2500 nm range ('880 patent at column 7, lines 40-42; see also column 28, lines 28-41).
The effect of pH on the spectrum of methemoglobin (MetHb) is known and has been reported in the literature. See Brunzel et al., "pH-Dependent Absorption in the B and Q Bands of Oxyhemoglobin and Chemically Modified Oxyhemoglobin (BME) at Low CL.sup.- Concentrations," Biophysical Journal, 49:1069-1076, 1986. The use of the MetHb spectrum for prediction of pH, however, is problematic since MetHb typically represent only about 1% of the total hemoglobin in blood.
The effect of pH on the spectrum of oxyhemoglobin (O.sub.2 Hb) has also been studied. See Brunzel et al., supra; and Wimberley et al., "Effect of pH on the Absorption Spectrum of Human Oxyhemoglobin: a Potential Source of Error in Measuring the Oxygen Saturation of Hemoglobin," Clinical Chemistry, 34:750-754, 1988. The focus of these studies, however, has not been on the use of an oxyhemoglobin spectrum to determine pH, but rather on obtaining a theoretical understanding of the chemistry of the heme group in the case of the Brunzel et al. work, and on controlling the effects of pH during the calibration of oxygen saturation meters in the case of the Wimberley et al. work. Neither reference in any way discloses or suggests the use of an oxyhemoglobin spectrum to non-invasively determine blood pH.